Travel North East of Riften with the Orc leader and fight your way until you get a proposition: tell the orc to get the mace from the Giant himself, or let him bribe you. If you make the orc kill the giant he will ultimately fail and you will have to slay the giant. If you accept the bribe, you must kill the giant and bring the mace back to the orc leader where he will turn on you (trust issues obviously).

When you return talk to Atub and Malacath will instruct you to place the mace on the altar. When he is done talking the mace will be transformed into Volendrung (2h mace) that you can take off the Altar.

Note that some players have had difficulty entering the Largashbur compound if Ugor, who would otherwise unlock the front gate, dies during the initial approach to the fortress. However, approaching any other Orc fortress such as Dushnikh Yal just southeast of Markarth will open up a miscellaneous objective to retrieve a set of Orcish gloves called The Forgemaster's Fingers. Once returned to the quest-giver, you will be named Blood Kin of the Orc tribes and have access to any of the Orc fortresses across the map.